Functional analyses of putative PalS (Palindromic Self-recognition) motifs in bacterial Hfq.

The bacterial protein Hfq has been linked to nucleic acid metabolism and signaling, however its explicit role has been elusive. Recently it was proposed that the C-termini of Hfq subunits in Hfq6 complexes could be involved in functional interactions with other Hfq hexamers and/or nucleic acids. To test the proposed model of the native Hfq complex experimentally, we genetically engineered chimeric Hfq6 complexes, in which C-termini of bacterial Hfq subunits were substituted with a sequence derived from human histone H2B (hH2B) that includes multiple functionally significant amino acids whose modifications have been linked to carcinogenesis. We demonstrate that this substitution results in an enhanced formation of dodecameric assemblies by the Hfq-hH2B hybrid - a result pointing to the possibility of a (functional) homology between these motifs in proteins from distant kingdoms. We hypothesize that these putative Palindromic Self-recognition (PalS) motifs could act as proteins' 'cohesive ends' that could allow the protein complexes carrying such motifs to interact dynamically and dissociate-reassociate in response to stress and/or growth phase-specific changes. We provide experimental support to the latter hypothesis and demonstrate that in E. coli the dodecameric Hfq assemblies are formed in a growth stage-specific manner. We describe a refined system - consisting solely of purified Hfq, polynucleotide phosporylase (PNP) and ADP - that allows reconstitution in vitro of characteristic 'SDS-insensitive' Hfq6-Hfq6 assemblies observed in experiments with whole-cell extracts obtained from exponentially-growing cells. We also optimized conditions for the extraction of intact native dodecameric Hfq complexes.